Presently, two types of spinal anesthesia techniques are routinely employed in surgical and/or child birth procedures. The two techniques are epidural anesthesia and subarachnoid anesthesia. With epidural anesthesia, a catheter is usually placed in the spinal epidural space and anesthesia is administered through the catheter. This technique has the advantage of providing for administration over an extended period of time, however, there are certain disadvantages related to the administration of anesthesia into this space. One such disadvantage is the non-uniform and often unpredictable distribution of the anesthesia. This distribution problem is due to the nature of the tissue in the epidural space, which is a tissue composed of primarily fatty and fibrous materials rather than free-flowing fluid.
A number of systems have been suggested for placing catheters, including some used for epidural anesthesia. U.S. Pat. No. 4,645,491 (Evans), discloses one such system particularly designed for spinal injection. Also refer to U.S. Pat. Nos. 3,672,367 (Scislowicz); 3,792,703 (Moorehead); 4,068,659 (Moorehead); 4,406,656 (Hattler et al); and 4,529,399 (Groshong et al), all disclosing devices for placing catheters wherein a stylet is position within the catheter and the catheter is positioned within a piercing needle. In U.S. Pat. No. 3,792,703 (Moorehead), for example, a system is disclosed where an intravenous catheter is introduced through a larger introducer needle. Within the bore of the catheter is a rigid tube which can be advanced with the catheter upon insertion. In U.S. Pat. No. 4,068,659 (Moorehead) an un-hubbed intravenous catheter with a conventional stylet is passed through a sheath and an introducer needle. The stylet has a distal wing-tip to aid catheter passage. The catheter is adapted to later receive a needle hub.
Because the subarachnoid space is far more liquified, and thereby a faster, more uniform and more predictable distribution medium, catheter delivered administration of anesthesia directly into the spinal subarachnoid space would be preferable for most surgeries and child birth procedures, were it not for one major side effect that is often associated with such administrations. The major problem is the severe post-operative headaches that often result from the puncture of the dural membrane (or dura), the barrier or membrane surrounding the spinal subarachnoid space, upon entrance of the relatively large conventional catheter-delivery anesthesia devices into the subarachnoid area.
Studies have been conducted which correlate the size of the puncture in the dural membrane to the severity of the headaches experienced by patients. In J. J. Bonica, "Postspinal Complications", Obstetrical Anesthesia: Current Concepts and Practice, S. M. Shnider, ed. Williams & Wilkins, Baltimore (1970) at page 175 the incidence of postpuncture headache was correlated to both needle size and bevel. Earlier, Krueger had correlated headache incidence with needle tip configuration for 20 gauge needles (beveled vs. pencilpoint). Krueger, "Etiology of Treatment of Postspinal Headaches", Current Researches in Anesthesia and Analgesia, pp 190-198 (May-June 1953). It is for these reasons that when epidural catheters are placed, efforts are normally taken to ensure that they do not rupture this dural membrane. Such efforts include, for example, using bent epidural introducer needles which tend to direct the catheter away from the dura, thereby minimizing the probability of puncture. See also Vandam et al, "Long-Term Follow-Up of Patients Who Received 10,098 Spinal Anesthetics", J.A.M.A. 161(7):586-591 (1956).
Thus, when subarachnoid anesthesia is employed, a catheter-based system is generally not used. Instead, a smaller straight needle is passed through the epidural space and the dural membrane into the cerebrospinal fluid, and a bolus injection of anesthesia is administered. Various methods for introducing small needles to deliver subarachnoid anesthetic have been suggested. In Harris et al, The Comparative Incidence of Postlumbar Puncture Headache Following Spinal Anesthesia Administered Through 20 and 24 Gauge Needles", Anesthesiology 14:390-397 (July, 1953) improvement was noted when a smaller 24 gauge needle was used instead of a 20 gauge needle. Harris et al employed a technique where a three and one half inch 24 gauge needle was inserted through a 2 inch 20 gauge introducer.
Greene, "A 26 Gauge Lumbar Puncture Needle: Its Value in the Prophylaxis of Headache Following Spinal Analgesia For Vaginal Delivery", Anesthesiology, 11:464-469 (July, 1950) described a similar "double needle" technique using a 26 gauge 10 cm. long spinal needle introduced through a 21 gauge 5 cm. long needle, both needles complete with their own stylets. According to Greene's technique, the 21 gauge needle with its stylet in place is introduced through the subcutaneous tissues until a plane of increased resistance is felt. The point of the needle is then reported to be within 1 cm. of the dura. The stylet is removed and the 26 gauge needle, with or without its wire stylet in place, is passed through the introducer to puncture the dura mater easily. The results of this procedure are said to prevent the high incidence of postpuncture headache encountered in the use of spinal analgesia for vaginal delivery.
A number of different needle guides and introducers are currently available for use in introducing small (25-26) gauge needles. These introducers aid in the direction of the needles, and reduce the incidence of coring and of introducing pieces of epidermis or bacteria into the subarachnoid space. The needles themselves are available with various tip configurations, non-cutting bevels or pencil point types being generally preferred.
Obviously, one disadvantage of using a needle injected bolus of subarachnoid anesthesia is that it lasts generally no more than a few hours. If the procedure lasts longer than the anesthetic bolus, spinal anesthesia is generally abandoned and resort is made to another anesthetic process.
The ideal delivery technique, therefore, would be one which provides for intermittent or continuous delivery of anesthesia, while at the same minimizing the degree and incidence of dural membrane rupture.
Recently, the use of a relatively small subarachnoid needle internally threaded with an even smaller (32 gauge) catheter was reported for administration of subarachnoid anesthesia. Hurley et al, "Continuous Spinal Anesthesia With a Microcatheter Technique", ASRA 12th Annual Meeting Abstracts, 12(1):53-54 (January-March 1987); Hurley et al, "New Microcatheter Technique Reduces Anesthetic Needs", Anesthesiology News, pp. 12-13 Oct. 1987. Using this technique, the needle is used to puncture the dura, and a catheter is inserted through the inner bore of the needle into the spinal subarachnoid space. The needle is then withdrawn leaving the catheter in place. There are, however, several problems with this insertion technique. For one, the catheter tends to fall out of the hole which is created by the larger needle since the dural membrane is not sufficiently elastic to return in part to its original position and hold the smaller catheter firmly in place. Secondly, the size of the catheter employed inside the needle is so small that it easily develops kinks. In addition, the small size of the needle results in rates of administration that are only between about 1/2 to 1 cc per minute. Since the normal dose of spinal anesthetic is approximately 1 to 3 cc, an anesthesiologist with excellent hand strength (for forcing the syringe) would thus be required to spend up to three minutes on administering the anesthesia alone. In essence, this system is merely a miniaturization of existing epidural catheter placement techniques, and one that does not adequately solve the unfulfilled and long-felt need for a better subarachnoid anesthesia technique.
Although unrelated to catheters used for administration of anesthesia, techniques are known for introducing catheters into vascular systems. One such technique is commonly used for introducing intra-venous (IV) catheters. Under this system, the catheter is positioned around the outside of a needle, and terminates in a taper immediately behind the tip of the needle at its distal end. At its proximal end a hub is employed to hold the catheter in place as the needle which it surrounds is withdrawn from the vein. The placement of IV catheters in veins which are quite close to the surface, thus requiring very little catheter passage through tissue before the needle and catheter tip enter the target vein, makes the concerns present there different from the concerns applicable to placement of catheters for spinal anesthesia.
New and better systems for supplying anesthesia, particularly supplying anesthesia to the spinal subarachnoid space, are needed. The present invention is directed to this end.